The subject invention refers to an electronic search system for a working tool, in which system a border cable, i.e. an electric cable, is placed above, under or on ground or floor, so that it separates an inner area within the border cable from an outer area, which working tool is preferably intended for attendance of ground or floor, such as grass-cutting, moss-scratching, watering, vacuum-cleaning, polishing, transportation etc., and a signal generator feeds the border cable with current, whose magnetic field affects at least one sensing unit located on the working tool, so that the sensing unit emits signals to a control unit, which in cooperation with an engine control, or a signal system for a driver, and at least one driving source directs the tool""s movement in order to prevent it from remoting from the inner area.
The idea to create a working tool, which manage completely by itself, such as a robot lawn mover or a robot vacuum-cleaner, is old. However, it has taken a long time before such kind of tool has reached the market. The solar cell driven lawn mover, called Solar Mower, is an example of that kind of product. It cuts the grass within a border cable, which has been placed in order to fence off the cutting area. Preferably the border cable is excavated into the ground. A signal generator feeds the border cable with current, whose magnetic field affects a sensing unit on the working tool. The sensing unit detects the intensity of the magnetic field and this intensity will increase when the working tool is coming closer and closer to the border cable. The microcomputer in the lawn mover is so programmed that the lawn mover reverses when a certain signal intensity has been achieved during the increasing of the signal intensity that occurs when the sensing unit is approaching the border cable. The lawn mover will as said move backwards from the border cable and then turn and begin to cut in a direction away from the border cable. Consequently, the lawn mover turns when the powerful magnetic field at the border cable has caused a signal intensity of a certain degree. On the other hand, the microcomputer cannot in any way separate the magnetic field on the outside from that on the inside of the border cable. It means that if the lawn mover should slide outside the border cable, or be pushed outside the border cable, the lawn mover will remote from the border cable in the wrong direction, i.e. out from the cutting area. However, it stops after approximately 4 meters when the signal intensity has dropped too much. These and other disadvantages are described in closer detail in a not yet published swedish patent application 9703399-7. This application refers to an electronic bordering system and describes in full detail the technology of how to separate an outer area from an inner area, thus essentially eliminating the above mentioned disadvantages. On the other hand, by way of this system no additional area within the inner area can be separated. For example, this would be desirable if you wish to cut a certain surface area especially carefully, or, if you wish that the tool shall stay within a certain area during the night. Also, it might be desirable to separate a special area for use in connection with a docking station for automatic battery charging.
The purpose of the subject invention is to substantially reduce the above outlined problems.
The above purpose is achieved in that the electronic search system in accordance with the invention is having the characteristics appearing from the appended claims.
The electronic search system in accordance with the invention is thus essentially characterized in that the first signal generator feeds the border cable with current containing at least two components of alternating-current with different frequency, and the components are lying in a known relation of time to each other, e.g. a regularly varying time relation, and that at least one more cable, called search cable, is placed at least partly within the inner area, so that it separates at least one search area within the inner area, and each search cable respectively is fed by a signal generator with an adapted current whose alternating-current components are virtually identical with the alternating-current components in the border cable, but where the direction of flow, at least for the alternating-current component with the higher frequency, in each of the search cables is alternating in time in being either in phase or out of phase in relation to the current flow direction in the border cable, so that the magnetic fields in the different areas, which are separated by each cable respectively, are forming at least three essentially unique time patterns, and hereby the control unit can evaluate the difference in the signals caused by the magnetic field""s different time patterns in the inner area, the outer area and at least one search area, and the control unit can therefore emit an area signal, which mainly takes up one of at least three states depending on the position of the sensing unit in relation to the border cable or each search cable respectively, i.e. an outer area state, an inner area state or at least one search area state.
By means of the specific current that is fed onto the border cable, and by the adapted current that is fed onto one or several search cables, at least three different areas can be separated, i.e. an outer area, an inner area and at least one search area. Owing to the fact that at least one more area, called search area, is added comparing with the above mentioned electronic bordering system, a number of additional possibilities are created. One or several search area/s could for instance be cut especially carefully in that the cutting tool remains for a longer time within this/these certain area/s. The tool could stay in a certain search area during the night. By way of a special xe2x80x9cfollow the cablexe2x80x9d mode the tool can move on along a search cable to a docking station for automatic battery charging.
The possibility for the control unit to evaluate if the sensing unit is located inside or outside the border cable is created in that the signal generator feeds the border cable with current containing at least two alternating-current components of different frequency, and in that the components are lying in a known relation of time to each other. Furthermore the frequences can preferably consist of multiples of each other, preferably equal number multiples of each other, and preferably the alternating-current components should stay in an essentially permanent time relation to each other. In order to increase the safety of the bordering system preferably an analogue signal is used, a so called quality signal, whose signal intensity is a measure of the intensity of the incoming signals of the control unit. Owing to this the tool can be shut off when the signal intensity is riskfully low. These and other characteristics and advantages of the invention will become more apparent from the detailed description of various embodiments with the support of the annexed drawing.